Topic 8: Energetics

Question 1

what are standard conditions

Answer 1

pressure of 100kPa

temperature of 298k

Question 2

define enthalpy change

Answer 2

heat energy change measured at constant pressure

Question 3

bonds breaking and bonds forming

Answer 3

bonds breaking= energy in

bonds forming= energy out

Question 4

define enthalpy of reaction

Answer 4

enthalpy change that takes place when the number of moles of substances in the equation react at a temp of 298k and pressure of 100kPa

Question 5

define enthalpy of formation

Answer 5

enthalpy change that takes place when one mole of substance is formed from its elements in their standard state at a temp of 298k and a pressure of 100kPa

Question 6

define enthalpy of combustion

Answer 6

enthalpy change that takes place when one mole of substance is completely burned in oxygen at a temp of 298k and pressure of 100kPa

Question 7

define enthalpy of neutralisation

Answer 7

enthalpy change that takes place when one mole of water is formed in a neutralisation reaction of 298k and pressure of 100kPa

Question 8

what are the two main reasons for an inaccurate value for an enthalpy of combustion

Answer 8

heat loss to the surroundings -> add a lid

incomplete combustion

Question 9

define Hess’ law

Answer 9

enthalpy change independent of route taken

Question 10

what is meant by bond enthalpy

Answer 10

the heat energy obtained when a bond is formed, and taken in when it is broken

Question 11

write an equation representing the bond enthalpy of O-H In water

Answer 11

H₂O-> HO + H

all (g)

Question 12

write an equation representing the mean bond enthalpy of O-H in water and use it to explain how mean bond enthalpy is different to bond enthalpy (3)

Answer 12

0.5H₂O -> 0.5 O + H
all (g)
idea of average taken of both O-H bonds

Question 13

The standard enthalpy change for the reaction between 1.8g magnesium and 25 cm3 of copper sulphate solution can be determined by mixing them in a glass beaker.
describe how the experiment would be carried out.

Answer 13

pipette the 25cm3 of copper sulphate into beaker
measure initial temp of solution
add 1.8g of magnesium and STIR
measure final temp

Question 14

The enthalpy change of a reaction measured is much less exothermic than the standard data book value. give 3 reasons why

Answer 14

heat loss to surroundings
reactions is complete
specific heat capacity of solution is approximate

Question 15

when the standard enthalpy of combustion of propan-2-ol is measured experimentally, the value obtained is always less negative than -2006kJ mol-1 even when heat loss to the surroundings and incomplete combustion have been accounted for. explain why.

Answer 15

water is produced as a gas (instead of liquid)
conversion of liquid to gas requires energy
measured values is less exothermic/ less energy is released

Question 16

write an equation for the standard enthalpy of formation of water, including state symbols.

Answer 16

H₂ (g) + 0.5 O₂ (g) -> H₂O (l)

Question 17

write an equation, including state symbols, for the standard enthalpy of formation of combustion of hydrogen

Answer 17

H₂ (g) + 0.5 O₂ (g) -> H₂O (l)